MAGNETIC DECLINATION AT BOMBAY. 
379 
series ( y ) which expresses it. Now, on a single glance at the three curves, it is perceived 
that (speaking generally) the points 1, 2, 3, See. to 12 circulate in each case in a left-handed 
direction about the points © r, the equinoctial months (3 and 4) and (9 and 10) lying 
on opposite sides of the closed curves. Moreover, the half-yearly points n and s and a and d 
lie in the same order about © r ; and the interval between the latter two, though smaller, 
is not inconsiderable in comparison with the interval between the two former points ; and 
the relation of the four points to each other is much the same in each of the three figures, 
the straight line joining a and d approaching a direction at right angles to the line of 
junction of n and s. This would seem to imply that the summer form of the diurnal 
variation merges into the winter form, not solely by the gradual fading away (and inver- 
sion at other places) of the former, in which case the diurnal variation at the opposite 
equinoxes would be the same, but also partly by the superposition of a distinct variation, 
whose turning-points are considerably removed from those of the ordinary diurnal vari- 
ation, and the extent of which is much more limited, and which moreover has an inverse 
character at the opposite equinoxes, at which times its influence on the ordinary mean 
diurnal variation is most sensible. It does not imply a retrograde rotation of the hours 
of maxima and minima accompanied by a gradual fading away of the variation, the vari- 
ation still preserving a constant character in all respects except extent and turning-points, 
because in that case, whilst a single rotation occurs in the arrangement of the points 
1, 2, 3, &c. to 12 in figure 31, there should be two and three rotations respectively in 
figures 32 and 33, and, in fact, there is only a single rotation in each of the three curves. 
Now, on referring to plate xxv. of the Philosophical Transactions of the Eoyal Society 
for 1863, where appear similar figures representing (in part) the diurnal variations of 
declination at Toronto and St. Helena, it is seen that these figures possess the character- 
istics just described of those for Bombay, with the exception that the rotation in the 
St. Helena curve is in a right-handed instead of a left-handed direction. To examine 
now more directly whether there be a semiannual inequality in the diurnal variation of 
declination having opposite characteristics at the opposite equinoxes, and at the same time 
to eliminate that other semiannual inequality whose times of opposition are the solstices, 
the mean diurnal variation was calculated for the half year January to June, and com- 
pared with the same for the half year July to December; half the excess, for each hour, 
of the former half year over the latter was taken to represent the inequality sought, 
which proved to be of so definite a character, and so similar for Bombay and Toronto, 
as to tend to the extension of the examination to all other stations for which reduced 
observations were accessible. Distinct similarity was then found to exist in the curves 
representing the inequality at all the north-latitude stations, Toronto, Kew, Greenwich, 
Pekin, and Bombay, whilst curves of form approaching to an inversion, with some modi- 
fications of the above, was found for the south-latitude stations, St. Helena and the Cape 
of Good Hope. Table XVI. and curves Nos. 37 to 43, Plate XXX., show the character 
of this inequality for each station. 
3 e 2 
